In the satellite earth station, a power amplifier for amplifying a transmission power is used. In recent years, however, in place of a traveling wave tube amplifier (TWTA) which uses a traveling wave tube as one type of vacuum tube for the power amplification unit of the power amplifier, a solid state power amplifier (SSPA) which uses a field effect transistor (FET) is becoming popular. This is because a power amplification FET has an improved efficiency and increased power. Especially, along with the emergence of an FET using a gallium nitride (to be referred to as GaN)-based material (GaN HEMT: gallium nitride high electron mobility transistor), the transmission output of the SSPA has significantly improved.
It is well known that although the GaN HEMT is effective for power increase of the SSPA, it has a problem with the distortion characteristics. One of the distortion characteristics is small signal suppression due to the memory effect of the drain or gate bias of the FET.
In an application for satellite communication such as satellite news gathering (SNG), the GaN HEMT adopts a system which commonly amplifies a high power TV wave as video contents to be relayed, and a small signal of order wire (OW) wave as a link line of voice between an outside broadcast van and a studio of a TV station. If, therefore, small signal suppression occurs, the level of the OW wave decreases due to ON/OFF of the TV wave, and the line of the OW wave may be disconnected. Thus, it is necessary to prevent small signal suppression from occurring as much as possible.
Furthermore, a similar problem arises as wideband/multilevel modulation communication advances.